This invention relates to a bonded cathode and electrode structure for microwave triode tubes, a method of manufacture, and more particularly to a structure and method using boron nitride insulation bonded between a cathode and a control grid.
The grid-controlled power amplifier has long been useful for a variety of microwave applications. The L-64 and L-67 types, developed by J. E. Beggs and his associates as a consequence of work sponsored by the U.S. Army Electronics Command, have extended the range of performance of such devices. These advances were attained through the use of a closely spaced grid-cathode structure operating in the high-vacuum environment of a titanium-ceramic tube structure.
The construction of grid-cathode units with even closer spacing of grid and cathode and capable of high grid dissipation was continued using a grid and a heater which are rigidly bonded to the cathode by an insulating film. Boron nitride (BN) was identified as the preferred insulating material. Chemical vapor deposition (CVD) of BN was developed, and grid patterns with detail as small as 0.002 inch were formed by erosion through a mask with air driven Al.sub.2 O.sub.3 particles. The d-c characteristics of bonded grid tubes showed a high utilization of emission as useful plate current, ability to withstand large positive grid bias, and the option of a high level of current collection or a wide grid-anode gap. See U.S. Pat. Nos. 3,599,031; 3,638,062; and 3,694,260 by J. E. Beggs.
Several significant technical problems remained, potentially blocking the successful development of still further improvements at higher microwave frequencies of a bonded grid triode. These were:
A continuous buildup of nitrogen gas within the tube when bonded grid-cathode structures were operated at 1050 degrees C. Tube characteristics were degraded in less than an hour of continuous operation.
Degradation of the grid-cathode and heater-cathode resistances by a factor of 1000 in about thirty hours of operation.
Lack of a process for forming grid openings with dimensions as small as 0.001 inch without either undercutting the supporting insulation or shorting out the insulating layer with metal.
A method of removing photo resist in a partial pressure of a gas, which may be hydrogen, at about 100 degrees C. is shown in U.S. Pat. No. 3,837,856. Other U. S. patents on removing resist are U.S. Pat. Nos. 3,787,239; 3,582,401; 3,458,312; and 3,676,219.